The present invention relates to apparatus for sensing the torsional displacement and speed of rotation of a shaft and, more particularly, to sensing the torsional displacement of a shaft under torque loading as an indication of the applied torque as well as the speed of rotation of the shaft.
Various devices and methods are known for quantitatively determining the torsional displacement, or twist, of a shaft under load as an indication of the applied torque. When torque is applied to a non-rotating shaft, the torsional displacement can be simply measured as a function of the relative torsional displacement, or twist, of one end of the shaft relative to the other. The measurement of torsional displacement is more complicated in dynamic situations where the shaft is rotating at relatively high speeds. Dynamic situations requiring calculation of the torsional displacement include, for example, turbo-machines, such as aircraft gas turbines and other turbo-shaft engines. In these devices, a compressor is located at the forward end of the engine and is connected through a main shaft to a turbine at the rear end of the engine. The torsional displacement of the main shaft is sensed as an incident to measuring engine torque, and the quantitative result is then available as an indication of engine power.
In traditional torque sensing systems for shafts, magnetic sensors are positioned at opposite ends of the engine main shaft and respond to respective toothed wheels secured to the shaft with each sensor providing an electrical pulse output at a pulse repetition rate that varies with the speed of rotation of the shaft. As the shaft is subjected to varying levels of torque, one end of the shaft is torsionally displaced, or twisted, relative to the other end. This causes a change in the phase relationship between the pulse trains from the magnetic sensors. Evaluation of the change in the phase relationship between the pulse trains allows an accurate determination of applied torque and the pulse repetition rate also allows an accurate determination of shaft speed.
In tubrbo-shaft engine applications, main shaft deflection and speed are determine by a monopole torque sensor of the type disclosed in commonly assigned U.S. Pat. No. 4,602,515 to Eichenlaub and U.S. Pat. No. 4,488,443 to Parkinson. In the disclosed structure two toothed wheels are positoned adjacent one another on the engine shaft with one of the wheels secured to the shaft and the other of the wheels secured to the end of a hollow reference sleeve. The opposite end of the reference sleeve is connected to the shaft so that torsional twisting of the shaft will cause a relative rotational displacement between the two wheels. A single magnetic pick-up provides an output signal representative of the relative position of the wheels.
While magnetic sensing systems have been developed to a relatively reliable state, they represent a comparatively expensive instrumentation system. Since the sensing system operates in temperature ranges that vary from ambient temperature at engine start-up to 1500.degree. F. or more, the sensors must be designed with heat-resistant materials, and the electrical response characteristics as a function of temperature must be known to provide an accurate output for all operating temperatures. Additionally, magnetic sensing systems are susceptible to electro-magnetic interference (EMI) which can interfere with the correct output of the sensors. While EMI can be reduced with shielding, this solution adds considerable weight to the system and is particularly disadvantageous in airborne applications.